THIRD PERIOD OF DEVELOPMENT. 121 



ment is not yet so important, but only begins in the 

 development, and is only completed simultaneously 

 with the differentiation of these mesoblastic somites 

 The originally symmetrical foundations always 

 experience simultaneously with the differentiation 

 such a movement that the alternation of the meso- 

 blastic somite boundaries is restored. 



NEURAL CAXAL. 



In the living object the closing of the neural canal 

 may be recognised by the appearance of a contigu- 

 ous layer of the outer surface, at first very thin, 

 which closes the canal, thus converting it into a tube. 

 Further, in the living object, if the growth be very 

 marked, it may be seen that the cells of the neural 

 canal, just as the outer epiblast cells, are ciliated. 

 The long feeble cilia, whose direction is posterior, 

 may be followed from those stages in which the lumen 

 of the neural canal becomes distinct. Probably these 

 cells have kept their character as ciliated cells, even 

 during the invagination. The transverse sections pro- 

 vide us with more exact information with respect to 

 the closing of the neural canal. 



In the foregoing section of development the med- 



